Ultrafine fibers with a fiber diameter of 1,000 nm (=1 μm) or less as represented by a nanofiber that is defined to have a fiber diameter of from 1 to 100 nm have recently received attention as a subject to be studied. Specifically, investigations have been made into the use of ultrafine fibers for ultrahigh performance filters, separators of batteries, capacitors, and the like, grinding materials for hard discs, silicon wafers, and the like, and raw materials for high performance materials, because of their unusuality with respect to hygroscopicity, a tendency to absorb low molecular weight materials, and the like.
It is described that according to the system of extracting the sea component of fibers in a polymer alloy yarn, 60% or more of the island component domain is capable of producing ultrafine fibers having a diameter of from 1 to 150 nm (e.g., see Japanese Unexamined Patent Publication (Kokai) No. 2004-169261). However, because the island components are finely dispersed in the polymer alloy method (or incorporated spinning method), selection of two types or more of polymers that have solubility parameters (defined as (evaporation energy/molecular volume)1/2, also termed SP values) close to each other and that are incompatible is required. As a result, selection of the types of the polymers in accordance with the purpose, for example, making a polymer that forms the sea component and a polymer that forms the same island components, and selection of the copolymer components and physical properties such as an intrinsic viscosity cannot be made optionally. Moreover, because the islands-sea boundary area is significantly increased, a Barus phenomenon in which polymer flows after injection from the spinneret expands takes place. As a result, spinning stability-related problems such as formation of foreign materials on the face of the spinneret and poor stringiness arise. Furthermore, uniformity of the island diameter is far from being termed uniform as observed in the figures in Japanese Unexamined Patent Publication (Kokai) No. 2004-169261, and production of ultrafine fibers at the nanolevel as filaments yarn and short fibers having a uniform length has been impossible.
On the other hand, an electrospinning method of obtaining a fiber having a diameter of from a few nanometers to a few micrometers is illustrated (e.g., see the specification of U.S. Pat. No. 1,975,504). The procedure for obtaining an extremely fine fiber comprises applying a high voltage of from 2 to 20 kV between the tip of a nozzle containing a polymer solution and a base plate, whereby a charged polymer is injected from the tip of the nozzle at the instant when the electric repulsive force exceeds the surface tension, and collecting the injected polymer on the base plate. However, the electrospinning method has the following problems: the polymer to be used is restricted to one that has good solvent having a boiling point near 110° C.; the resultant nanofiber has a problem of size uniformity (e.g., a fiber as thick as 1 μm or more in diameter is mixed in the nanofiber); because the melt viscosity is required to be low to a certain degree, a high strength fiber cannot be obtained. Furthermore, in order to produce the fiber in an industrial production amount by currently published production methods, a spinneret having multi-nozzles and a base plate having a significantly large plate area are required. In other words, many unsolved problems still remain. Still furthermore, production of a filaments yarn and production of short fibers having an optional length are impossible.
Other methods of producing an ultrafine fiber having a diameter of 1 μm or less include a melt blowing method comprising blowing a molten thermoplastic polymer with a high speed air flow to form a fiber, and a flash spinning method comprising injecting a polymer solution at the moment when the polymer solution prepared by dissolving a polymer in a solvent at high temperature and high pressure is made gaseous, to form a net-like fiber. However, as in the electrospinning method, these methods have the problem that the fiber diameter is not uniform and the problem that a filament yarn cannot be obtained (see, e.g., Basics and Applications of Nonwoven Fabrics P. 107-127 (1993), edited by the Textile Machinery Society of Japan).
Furthermore, it is well known that extremely fine fibers of island components can be obtained by extracting and removing the sea component of an islands-in-sea type composite spun fiber obtained by compositing at least two types of molten polymers within a spinneret. However, the lower limit of the fiber diameter is at most at the level of 2 μm (0.03 dtex for a poly(ethylene terephthalate)). Obtaining an island diameter of 1 μm or less has been extremely difficult (e.g., see “Newest Spinning Technologies” 215 (1992), edited by the Society of Fiber Science and Technology, Japan).
Accordingly, neither a method of producing ultrafine filament yarns having a fiber diameter of 1 μm or less and a uniform fiber diameter distribution nor a method of producing ultrafine short fibers having equal fiber lengths has been proposed.